Assessment of the Shear Bond Strength between Nanofilled Composite Bonded to Glass-ionomer Cement Using Self-etch Adhesive with Different pHs and Total-Etch Adhesive.

STATEMENT OF THE PROBLEM
In the sandwich technique, the undesirable bond between the composite resin and glass-ionomer cement (GIc) is one of the most important factors which lead to the failure of restoration. Total-etch and self-etch adhesives may improve the bond strength based on their pH.


PURPOSE
The purpose of this study was to evaluate the shear bond strength between the nanofilled composite resin and GIc using different adhesives.


MATERIALS AND METHOD
In this experimental study, 40 specimens (6×6mm) in 4 groups (n=10) were prepared in acrylic mold. Each specimen contained conventional GI ChemFil Superior with a height of 3mm, bonded to Z350 composite resin with a height measured 3mm. In order to bond the composite to the GI, the following adhesives were used, respectively: A: mild Clearfil SE Bond self-etch (pH=2), B: intermediate OptiBond self-etch (pH=1.4), C: strong Adper Prompt L-Pop (pH=1), and D: Adper Single Bond 2 total-etch (pH=7.2). The shear bond strength was measured by using universal testing machine with a crosshead speed of 1mm/min. One-way ANOVA and Tukey's test were used to analyze the data (p< 0.05).


RESULTS
The shear bond strength in group A was significantly higher than group B (p= 0.002), C (p< 0.001), and D (p< 0.001). Moreover, the shear bond strength of groups A and B (self-etch) was significantly different from group D (total-etch) (p< 0.001); and C (self-etch) with D (p= 0.024).


CONCLUSION
The results of this study showed that applying the mild self-etch adhesive between the composite and the GIc results in stronger shear bond strength compared to intermediate and strong self-etch adhesives. Moreover, the self-etch adhesive increased the shear bond strength between composite resin and GIc more significantly than total-etch adhesive.


Introduction
Laminate technique or sandwich restoration is one of the methods used in dental composite restoration, [1] in which two different materials namely glass-ionomer cement (GIc) and composite resin are used. In this technique, the GIc or resin-modified glass-ionomer cement (RMGIc) is placed between the dentin gingival margins and occlusal composite restoration. [2] The proper bond between GIc and resin composite is necessary for successful restoration. This method is mainly applied to benefit from both the physical and aesthetic properties of these materials. GIc presents two interesting features in restorations by bonding spontaneously to the dentin and releasing fluoride. [2] Some disadvantages of these materials include poor physical-mechanical properties and esthetics which can be compensated by the overlying composite resin. [3][4] Etching the GIc is effective to obtain the favorable bond of composite. [5] Using 35% phosphoric acid as surface treatment of GIc may increase the shear bond strength of this cement to composite resin. [6] The bond strength between the conventional GIc and composite resin is due to the porosity in the etched surface of GIc. [7] It has been found that in etching procedure, a 0.5 mm thickness of GIc and 20 seconds of etching is necessary to provide a proper bonding surface. [8] In order to reinforce the bond strength between the GIc and composite resin, surface treatment with selfetch system has been suggested. Since the self-etch system has less technique sensitivity, it can mostly meet the dentists' need for using sandwich technique. [9] The self-etch system can be either one-step or two-steps application procedure. Considering the invasion of selfetch adhesives, they are divided into strong, intermediate and mild versions. [10][11] It has been reported that higher acidity of the self-etch adhesive results in higher dentin demineralization. The strong self-etch adhesive has a pH of 1 or less. The self-etch with lower pH offers low bond strength, particularly in the dentin. The mild self-etch adhesive, however, generally has a pH of 2 and this low acidity causes a superficial demineralization, being less than 1 mm in dentin. [12][13] Previous studies revealed the self-etch adhesive provided higher shear bond strength between the RMGIc and composite resin than other adhesives. [14][15] In a study, Mount showed that the changes in the pH of the adhesive affected the bond strength between the GIc and composite resin. [16] It is reported that applying self-etch adhesive on the surface of the GIc before using the composite resin improved the bond strength. It also decreased the clinical time because of the synchronous penetration of the adhesive resin along the self-etch process. [17] Apparently, the bond strength between GIc and composite resin considerably affects the clinical success of esthetic restorations. Hence, the present study was designed to evaluate the shear bond strength of nanofilled composite bonded to conventional GIc by using self-etch adhesives with different pHs and total-etch adhesive.

Materials and Method
In this experimental study, 40 specimens of 6×6mm were prepared in 4 groups (n=10) in acrylic mold  phosphoric acid for 15 seconds, and was then rinsed. [6] After that, the total-etch adhesive was applied to the GIc surface by a microbrush. Finally, the composite was added to the samples as in other groups.
The samples were all stored in distilled water for 24 hours at room temperature. The shear bond strength was evaluated by the universal testing machine (Zwick/Roell Z020; Germany) at a crosshead speed of 1 mm/min ( Figure 2).
The obtained data were analyzed by using SPSS software, version 20. One-way ANOVA and Tukey test were used to compare the mean shear bond strength among the groups. The significance level was set at 0.05.

Figure 2:
Testing the shear bond strength by using universal testing machine.

Results
The mean±SD of shear bond strength of each group are presented in Table 2. The shear bond strength in group A (mild self-etch adhesive) was significantly higher than group B (p= 0.002), C (p< 0.001) and D (p< 0.001).
The maximum and minimum shear bond strength was obtained respectively in Group A with the mean of 7.77 MPa and group D (total-etch adhesive) with the mean of 3.45 MPa.
Comparing the groups by Tukey's test (   Figure   3 shows that the shear bond strength of the four tested groups.  adhesive. [15] The total-etch system needs two separate phases of rinsing and drying and has also a higher technique sensitivity. [18][19] Moreover, the GIc surface may become weak during the drying phase. [20] Owing to the acidic monomer in its composition, Overall, further studies are recommended to examine and evaluate the effect of different generations and the application of bonding with different pHs on the bond strength between the composite and light GI.

Conclusion
Concerning the limitations of this study, it can be concluded that using the mild self-etch adhesive (Clearfil